Biopolymer-based electromagnetic interference shielding materials

ABSTRACT

An electromagnetic interference (EMI) shielded device which includes an object to be shielded and an EMI shielding material encompassing the object. The EMI shielding material is made up of, but not limited to a broadband biopolymer or polymer dissolved in organic solvents, and metal and carbon-based nano-powders or nanoparticles. The specific makeup of the shielding material and fabrication procedure of the shielding material is also included herein.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 15/196,114, filed on Jun. 29, 2016, entitled BIOPOLYMER-BASEDELECTROMAGNETIC INTERFERENCE SHIELDING MATERIALS, which is a divisionalof U.S. patent application Ser. No. 13/746,993, filed Jan. 22, 2013,entitled BIOPOLYMER-BASED ELECTROMAGNETIC INTERFERENCE SHIELDINGMATERIALS, which in turn claims priority to and benefit of U.S.Provisional Application No. 61/588,981, filed Jan. 20, 2012, the entirecontents of which are incorporated by reference herein for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.FA-8650-10-0-C-5406 awarded by the U.S. Air Force. The government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

As the speeds at which electronics operates continue to increase, thestray high frequency radio frequency (RF) or electromagneticinterference (EMI) radiation that is emitted continues to increase aswell. This high frequency RF/EMI radiation interferes with all theelectronic devices, chips and circuit boards in close proximity to eachother and can cause the chips and circuits to malfunction. Thus, theelectronic devices, chips and circuit boards need to be shielded fromone another to prevent this stray radiation from causing malfunctions.This RF/EMI radiation can come from devices, chips, intra- and intercircuit boards and between boards. The higher the frequencies the moredifficult and expensive it becomes to shield the circuits. The prior andcurrent RF/EMI shielding technologies use traditional techniques basedon conductive materials or coatings, such as gold (Au), copper (Cu),silver (Ag) or aluminium (Al), surrounding the device, chip and/orcircuit board. This is a complicated, expensive and time-consumingprocess.

Prior procedures are difficult, complicated, labor intensive, expensiveand time-consuming, especially the electrical grounding processing.

In addition, the insulating materials or molds, which are deposited ontop of the device, chip or circuit to prevent electrical shorting,typically have low thermal conductivity. This holds the heat, generatedby the device, chip or circuit, in the device, chip or circuit.Retention of heat in the device, chip or circuit can cause overheating,reduce performance and reduce lifetime.

Another typical example is a conductive metal grid or mesh enclosuresurrounding the device, chip or circuit to block external static andnon-static electric fields. This is known as a Faraday cage. A Faradaycage shields the interior from external electromagnetic radiation if theconductor is thick enough and the holes of the grid or mesh aresignificantly smaller than the wavelength of the radiation. This type ofshielding is also rigid and non conformable. In addition, neither typeof shielding technique shields between devices or wires within a chip orbetween chips or wires within or on a board. They only shield frominternal radiation getting out or from external radiation getting in.

BRIEF SUMMARY OF THE INVENTION

The embodiments of this disclosure facilitate the fabrication andapplications of biopolymer-based, broadband electromagnetic interferenceshielding materials (BESM) that can be employed to numerous areasincluding, but not limited to, electronics, telecommunications, andaviation, etc.

Broadband biopolymer-based electromagnetic interference shieldingmaterials (BESM) are made up of, but not limited to, deoxyribonucleicacid (DNA), DNA lipid complex, silk, or any other biopolymer, whichcould be dissolved in organic or ionic solvents, and metal orcarbon-based nano-powders or nanoparticles. The lipid material could be,but not limited to, cetyltrimethylammonium chloride orhexadecyltrimethylammonium chloride (CTMA). The organic solvents couldbe, but not limited, butanol, ethanol, methanol, or a chloroform/alcoholblend. The metal and carbon-based nano-powders or nanoparticles couldinclude, but not limited to, noble metals (such as silver, gold,aluminum, copper), carbon-based graphite, carbon black, graphene,nanotubes, flakes, fibers or other conductive materials.

The BESM can be nonconductive, if the metal filler doping ratio is lowerthan the percolation threshold. The surface resistance of nonconductiveBESM films could be >30 MΩ/sq. and the BESM can form BESM films oncertain substrates and is spreadable, conformable and curable at low orroom temperature with good adhesion with most materials including, butnot limited, metal, glass, wood, plastics, semiconductors and otherbiopolymers.

The biopolymer-based material is not restricted to DNA-based and couldbe any organic polymers including, but not limited, Poly(methylmethacrylate) (PMMA) or Polyvinyl alcohol (PVA), polycarbonate, etc.

The BESM films have very have high EMI shielding efficiency overfrequency ranges from substantially DC (0 Hz) to 100 GHz. The shieldingefficiency, for example, could be as high as 60 dB for a thin BESM filmwith a thickness of ˜50 μm. For example, but not limited thereto, theBESM films provide efficiency of up to approximately 60 dB for shieldingmaterial with a thickness of approximately 50 mm over a frequency rangein the DC and Hz and MHz ranges and approximately 20-30 dB in the GHzrange.

Further, a thin BESM layer (typically ˜30-50 μm) could block EMIradiation up to, for example, 60 dB effectively over an RF frequencyrange from low kilohertz (KHz) to tens of gigahertz (GHz), exhibitingexcellent EMI shielding efficiency.

In addition, the method of making the EMI shielding material includes,but is not limited to the following steps: dissolving DNA in de-ionizedwater; dissolving CTMA in de-ionized water; making a DNA-CTMA complex;making a butanol based DNA-CTMA solution; and making the EMI shieldingmaterial in the form of, for example, a film from the butanol basedDNA-CTMA solution.

BESM also has the following advantages:

a. High EMI shielding efficiency.

b. Nonconductivity, making a single EMI shielding layer on top of, andin direct contact with, electronic circuit boards possible.

c. Spreadable (with liquid phase BESM), conformable and compatible withthe shapes and most object materials, such as metals, glass and plastic,etc.

d. Large thermal conductivity that could dissipate heat generated byhigh-speed electronics

e. Low cost for materials and processes, etc.

For a better understanding of the present invention, reference is madeto the accompanying drawings, detailed description and claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustration of a basic fabricationprocedures of the BESM (DNA-based in this example);

FIG. 2A is a set of typical measurement results of EMI shieldingeffectiveness of BESM at a frequency range from tens of KHz to hundredsof MHz;

FIG. 2B is a set of typical measurement results of EMI shieldingeffectiveness of BESM at a frequency range from 6 GHz to 16 GHz;

FIG. 3 is a set of typical measurement results of the EMI shieldingeffectiveness vs. film thickness in BESM samples;

FIG. 4 is a DNA-CTMA-based film's thermal properties, indicating thefilm could sustain and be stable above 230° C.; and

FIG. 5 is a schematic of a single layer EMI shielding coating on amicroelectronic printing circuit board (PCB).

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of this disclosure facilitate the fabrication andapplications of biopolymer-based, broadband electromagnetic interferenceshielding materials (BESM) that can be employed to numerous areasincluding, but not limited to, electronics, telecommunications, andaviation, etc.

Broadband (from DC (0 Hz) to approximately 100 GHz) biopolymer-basedelectromagnetic interference shielding materials (BESM) are made up of,but not limited to, deoxyribonucleic acid (DNA), DNA lipid complex,silk, or any other biopolymer, which could be dissolved in organic orionic solvents, and metal or carbon-based nano-powders or nanoparticles.The lipid material could be, but not limited to, cetyltrimethylammoniumchloride or hexadecyltrimethylammonium chloride (CTMA). The organicsolvents could be, but not limited, butanol, ethanol, methanol, or achloroform/alcohol blend. The metal and carbon-based nano-powders ornanoparticles could include, but not limited to, noble metals (such assilver, gold, aluminum, copper), carbon-based graphite, carbon black,graphene, nanotubes, flakes, fibers or other conductive materials.

The BESM can be nonconductive, if the metal filler doping ratio is lowerthan the percolation threshold. In a mixture between a dielectric and ametallic component, the conductivity and the dielectric constant of thismixture show a critical behavior if the fraction of the metalliccomponent reaches the percolation threshold. The behavior of theconductivity near this percolation threshold will show a smoothchangeover from the conductivity of the dielectric component to theconductivity of the metallic component. So, below percolation thresholdthe BSEM is non-conductive and above the percolation threshold the BSEMis conductive. The surface resistance of nonconductive BESM films couldbe >30 MΩ/sq. and the BESM can form BESM films on certain substrates andis spreadable, conformable and curable at low or room temperature withgood adhesion with most materials including, but not limited, metal,glass, wood, plastics, semiconductors and other biopolymers.

The biopolymer-based material is not restricted to DNA-based and couldbe any organic polymers including, but not limited, Poly(methylmethacrylate) (PMMA) or Polyvinyl alcohol (PVA), polycarbonate, etc.

The BESM films have very have high EMI shielding efficiency overfrequency ranges from substantially DC to 100 GHz. The shieldingefficiency, for example, could be as high as 60 dB for a thin BESM filmwith a thickness of ˜50 μm. For example, but not limited thereto, theBESM films provide efficiency of up to approximately 60 dB for shieldingmaterial with a thickness of a approximately 50 mm over a frequencyrange in the DC and Hz and MHz ranges and approximately 20-30 dB in theGHz range.

Further, a thin BESM layer (typically ˜30-50 μm) could block EMIradiation up to, for example, 60 dB effectively over an RF frequencyrange from low kilohertz (KHz) to tens of gigahertz (GHz), exhibitingexcellent EMI shielding efficiency.

BESM also has the following advantages:

a. High EMI shielding efficiency.

b. Nonconductivity, making a single EMI shielding layer on top of, andin direct contact with, electronic circuit boards possible.

c. Spreadable (with liquid phase BESM), conformable and compatible withthe shapes and most object materials, such as metals, glass and plastic,etc.

d. Large thermal conductivity that could dissipate heat generated byhigh-speed electronics.

e. Low cost for materials and processes, etc.

The present embodiments utilize non-conductive, biopolymer-based, metalor carbon nanoparticle or nanopowder composite, EMI shielding material(BESM), and may involve the following process: cast the BESM on theboards to form a single layer coating. The coating could be cured at lowor room temperature, for example, approximately 60-80 degrees F. for afew hours, for example, approximately 1-4 hours. There is no need forgrounding, since the coating is nonconductive Another importantadvantage of using nonconductive BESM for EMI-shielding on high-speedelectronic devices and PCBs is that the BESM coating could quicklydissipate the heat generated by the high-speed electronics, due to itslarger thermal conductivity compared to other polymers and epoxies. Forexample, thermal conductivity was measured of 0.82 watts/meter, degreesKelvin (W/(m, K)) for DNA only (no nanoparticles of nanopowders) and0.62 W/(m, K) for DNA-CTMA only (no nanoparticles of nanopowders). Thethermal conductivity of PMMA is 0.12 W/(m, K). Therefore, the thermalconductivity of the DNA-based materials is a minimum of five to seventimes higher than that of other polymers and epoxies.

It is also possible with the present embodiments to harden electrical,electronic devices, systems, transformers and transmission lines.

Further, for sensors applications, the BESM can fit in the sensorsapplications under an environment where radio frequency (RF) is present.An excellent example of such applications is the hyperthermia usingmicrowave treatment for cancer. It was found that cancer cells are verysensitive to temperature. By heating the cancer cells, it may kill thetumors. It has been shown that the cancerous tissues can be destroyed atexposures of ˜108° F. for one hour. Hyperthermia treatment of cancer isbased on this. In the hyperthermia microwave treatment, the temperaturearound the cancer areas can be raised using RF radiation, similar tothat of microwave ovens used in our kitchens to heat food. Preciselycontrolling temperature is the key. To monitor the temperatureprecisely, expensive dielectric and optical fiber-based temperaturesensors are most commonly used since inexpensive, metal-basedtemperature sensors will disturb the RF waveforms and also, loosemeasurement accuracy. Coating BESM on to the metal-based temperaturesensors can avoid such problems and reduce the cost of the treatment.

The broadband, biopolymer-based EMI shielding materials (BESM) can beproduced by mixing, for example, DNA-lipid complex (DNA-CTMA) andmetal-based nanoparticles or nanopowders as fillers, where CTMA is anabbreviation of cetyltrimethylammonium chloride orhexadecyltrimentylamonium chloride.

The process procedures are described with respect to FIG. 1. Thedetailed process for a DNA-lipid-based BESM material is described asfollows;

Make Water-Based DNA Solution

Dissolve DNA in de-ionized water at room temperature, for example,approximately 60-80 degrees F. at a ratio of approximately 4 grams/literusing a magnetic stirrer. Mix until fully dissolved which may takeapproximately two hours;

Dissolve CTMA in deionized water at approximately room temperature, forexample, approximately 60-80 degrees F. at a ratio of approximately 4grams/liter using a magnetic stirrer. Mix until fully dissolved whichmay be approximately five minutes;

Make DNA-lipid complex, DNA-CTMA. The DNA solution is added drop-wise tothe CTMA solution with a burette at a rate of about 1 drop per minute. Awhite DNA-CTMA precipitate forms as the DNA is added to the CTMA. Thesolution is mixed for an additional 4 hours at room temperature. FilterDNA-CTMA precipitates out of the mixture and rinse thoroughly;

Place the DNA-CTMA precipitates in a beaker and then place in an oven at˜40° C. to dry the DNA-CTMA precipitates overnight, for example,approximately 8 hours;

Make butanol-based DNA-CTMA solution: Dissolve the DNA-CTMA precipitatein butanol at a ratio of about 10 weight percent of DNA-CTMA to butanolusing a tumbler or a magnetic stirrer. At room temperature, for example,approximately 60-80 degrees F. mix until completely dissolved which isfor approximately 6-8 hours. At 40 degrees C. the mixing time can bereduced to approximately 1 hour.

Making BESM Films

Make BESM by mixing metal nano-powders at a desired ratio (wt %,typically from 2-10 wt %) with the DNA-CTMA-butanol solution using astirrer or sonicator;

Cast BESM on substrates, such as glass or plastic slides and dry them atroom temperature to form BESM films on the substrates. BESM filmthickness is typically around 30-50 μm.

A wide selection of metal nanoparticle and nanopowder fillers have beentested for their performance in EMI shielding efficiency, including, butnot limited, silver, carbon-based graphite, graphene and othermetal-based nanopowders and nanoparticles;

BESM has been tested under a wide frequency range. FIGS. 2A and 2B showthe BESM test results for a frequency range of from tens of KHz tohundreds of MHz. SC1 and SC2 were DNA-CTMA-silver nanopowder-based BESMsamples on scotch tape with silver-doping ratios of 4% and 8% andthicknesses of 30-50 μm, respectively.

BESM has been tested under wide frequency range. FIG. 3 shows the BESMEMI shielding effectiveness for a frequency range of from 7 GHz to 17GHz. The samples were DNA-CTMA-silver and silver-aluminumnanopowder-based BESM samples on 1 inch×3 inch glass slides with Ag andAg—Al-doping ratios of 5% each and thickness of 30-50 μm.

BESM has been tested over a temperatures range of −50° C. to +70° C. andover a frequency range of from 7 GHz to 16 GHz for EMI shieldingefficiency. The averaged results showed no significant changes.

DNA-CTMA possesses very stable thermal properties. FIG. 4 shows the filmcan sustain and be stable at temperatures as high as 230° C.

Non-biopolymer-based BESM materials, such as, but not limited,Poly(methyl methacrylate) (PMMA) and silver nanopowder were alsoformulated and could be used as a polymer matrix to replacelipid-DNA-CTMA.

In summary, novel biopolymer-metal and carbonnanoparticle/nanopowder-based EMI shielding materials (BESMs) haveproven very effective nonconductive materials for broadband and high EMIshielding efficiency. The DNA-lipid-based complex (DNA-CTMA) served asthe polymer matrix host, while the metal and carbon-based nanoparticlesand nanopowders served as the fillers or guests. BESM could be madehighly conductive or nonconductive (>30 MW/sq), depending on thenanoparticle and nanopowder doping ratios, whether one reaches thepercolation threshold or not. It was found that at certain dopingratios, between about 4% and about 8%, which are also lower than thepercolation threshold, the BESM films still possess very high EMIshielding efficiency while at the same time being nonconductive. Dopingratios for nanopowders are 4% to 8% to ensure material is nonconductive.

Embodiments of this invention including, but not limited to, theembodiments described above may be utilized as a single layer EMIshielding coating on microelectronics circuitry for intra- and inter-EMIshielding, as shown, for example, in FIG. 5.

Composite materials, such as polymer-matrices containing conductivefillers, are very attractive for shielding electromagnetic interference(EMI) due to their high shielding efficiency and seamlessness,processability, flexibility, light-weight and low-cost. Here, we reportthe development of novel, biopolymer-based EMI-shielding materials(BESMs), made up of DNA and metal and carbon nanoparticles andnanopowders. A thin DNA-based BESM layer (typically ˜30-50 μm) caneffectively block EMI radiation up to 60 dB over an RF frequency rangeof from KHz to tens of GHz, exhibiting excellent EMI shieldingefficiency. A wide selection of metal and carbon nanoparticle/nanopowderfillers for BESMs has been tested for their performance in EMI shieldingefficiency. Among them, silver and carbon-basednanoparticles/nanopowders have demonstrated the best performance andwere selected for further investigation. The silver-doped DNA-based BESMfilms could also be made non-conductive while their EMI shieldingefficiency was still well-preserved. The nonconductive BESM could have agreat potential in the microelectronics industries for EMI shielding onelectronic devices and circuit boards.

The present embodiments incorporate DNA-based biopolymer EMI-shieldingmaterials (BESMs), made up of DNA and metal nanoparticles andnanopowders. Doped with adequate metal and carbon-based nanoparticlesand nanopowders as fillers, the BESMs have exhibited excellent EMIshielding efficiency. It has been shown that a thin BESM layer(typically ˜50-100 μm) can block RF radiation up to 60 dB at frequenciesranging from KHz to tens of GHz. A wide selection of metal andcarbon-based nanoparticles and nanopowders have been tested for theirperformance in EMI shielding efficiency. For example, but not limitedthereto, silver and carbon have demonstrated excellent performance.Depending on the doping ratios, whether above or below the percolationthreshold, the BESMs could be rendered very conductive or nonconductive.Even the nonconductive silver-doped DNA-based BESM films have shown veryhigh EMI shielding efficiency. The nonconductive BESMs may have a greatpotential in the microelectronics industries for EMI shielding onelectronic devices and circuit boards.

EMI shielding refers to absorbing or re-directing (reflecting orscattering) the propagation of electromagnetic radiation by a materialas a shield.

a) Shielding by Reflection

For reflection of the radiation by a shield material, the material mustbe electrically conductive, i.e. it must have mobile charge carriers,(electrons or holes) which interact with the electromagnetic fields inthe radiation. Metals are the most common materials for reflection-typeshielding. The electrical conductivity of a shielding material needs notnecessarily high and a material with a volume resistivity in the orderof 1 Ω-cm could be good enough for effective EMI shielding.

b) Shielding by Absorption

For significant absorption of the radiation by the shield, the shieldshould have electric and/or magnetic dipoles that interact with theelectromagnetic fields in the radiation fields. The materials that havea high value for their dielectric constant could provide efficientelectric dipoles. It should be noted that the reflection losses decreasewith increasing frequency, whereas the absorption losses increase withincreasing frequency. Hence, EMI-shielding based on absorption would bemuch more effective at higher RF frequencies.

c) Shielding by Multiple Reflections (Scattering)

The mechanism of EMI shielding by multiple reflections, or scattering,is quite different than the single reflection shielding using conductivematerials. Multiple reflections usually refers to the reflections atvarious surfaces or interfaces throughout the shielding materials. Thismechanism requires the presence of a large effective internal surfacearea or interface area in the shield. An example of a shield with alarge internal surface area is a porous or foam material. Anotherexample (we will use it in our approach) is a material containingfillers which provide large internal surface areas. The losses due tomultiple reflections can be neglected when the distance between thereflecting surfaces or interfaces are much longer compared to the skindepth.

d) Skin Effect

Electromagnetic radiation at high frequencies penetrates only the nearsurface region of an electrical conductor. This is known as the skineffect. The electric field of a plane wave penetrating a conductor dropsexponentially with increasing depth into the conductor. The depth atwhich the field drops to 1/e of the incident value is called the skindepth (δ), which is given by

${\delta = \frac{1}{\sqrt{\pi\; f\;{\mu\sigma}}}},$where f is frequency, μ is the magnetic permeability, which equals μ0μτ(μ0=1.26×10⁻⁶ H/m and μ_(τ) is the relative magnetic permeability of thematerial compared with that of copper, which is 1), and σ is theelectrical conductivity in Ω⁻¹m⁻¹. Therefore, the skin depth decreaseswith increasing frequency and with increasing conductivity orpermeability.

e) Composite Materials for Shielding

Due to the skin effect, a composite material having conductive fillerswith small unit size is very effective since the filler's surface areasare large. Therefore, the smaller unit size of the fillers in thecomposite material, the better the effectiveness of the EMI shielding.

The advantages of DNA-based shielding materials include:

a) Low Cost

i. Low material cost. DNA is a byproduct of the fishing industry, suchas salmon milt and row sacs; therefore it could be very low cost.

ii. Low processing cost due the simple and low temperature processingprocedures.

b) Properties for High EMI-Shielding Effectiveness

i. Variety of electrical conductivity. Depending the properties andconcentrations of doping fillers, the DNA-based composite materialscould be very conductive or nonconductive. More interestingly, thenonconductive BESM could have very efficient EMI shielding. Such aunique property of BESMs could have many applications in themicroelectronics industry.

ii. Large dielectric constant of DNA. The dielectric constant of DNA hasbeen measured at 8 to 14 and could be made >400. A large dielectricconstant in the materials provides large amount of electric dipoles thateffectively absorb EM wave, enhancing effectively EMI-shielding.

c) Large Thermal Conductivity

Measuring the thermal conductivity of, for example, DNA of 0.82 W/(m, K)shows it to be approximately seven time higher than PMMA IS THISDEFINED?. A large thermal conductivity can dissipate heat quickly thatis very important for EMI shielding on high-speed electronic devices,which usually create quite an amount of heat when they are operated.

The BESM described herein is comprised of a DNA-cetyltrimethylammonium(CTMA) matrix material and metal and carbon nanoparticles/nanopowders(fillers). The materials and fabrication procedures are described asfollows.

DNA and DNA-CTMA: The raw DNA material (salmon sperm-based) was providedby Prof. N. Ogata, Chitose Institute of Science and Technology, Chitose,Japan. A DNA-lipid complex, namely DNA-CTMA was produced as the hostmatrix material in the experiments. DNA could be readily converted tothe DNA-CTMA complex by a cationic surfactant reaction. The resultingDNA-CTMA complex is only soluble in polar organic solvents, but notsoluble in water. The CTMA was purchased from Sigma-Aldrich.

Metal and Carbon fillers: A wide selection of metal and carbonnanoparticle/nanopowder fillers has been tested for their performance inEMI shielding efficiency.

Among them, silver and carbon-based nanoparticles and nanopowders havethe best performance and were selected for further investigation. Sometypical metal fillers used in our experience are listed in Table 1below:

TABLE 1 Metal fillers used in BESM Metal Powder Type Size Purity MakerAg Metal-Basis 0.7-1.3 μm 99.9% Alfa Aesar Brass Alloy ~44 μm 99.9% AlfaAesar Cu Metal-Basis 0.5-1.5 μm 99.0% Alfa Aesar Ag coated AlMetal-Basis ~50 μm 99.0% Alfa Aesar Fe Metal-Basis ~44 μm 98.0% AlfaAesar Graphite #1 Synthetic ~44 μm 99.9995%   Alfa Aesar ConductiveGraphite #2 Natural Crystal 2-15 μm 99.9995%   Alfa Aesar Ni Metal-Basis~50 μm 99.995%  Alfa Aesar Graphene Flakes 8 nm 99.9% Graphene Lab

Making DNA-CTMA solutions is described in greater detail above andsummarized below

i. Make water-based DNA solution: Dissolve DNA in de-ionized water at aproper ratio using a magnetic stirrer at room temperature for severalhours until DNA is completely dissolved;

ii. Dissolve CTMA in deionized water using a magnetic stirrer for a fewminutes until CTMA is completely dissolved;

iii. Make DNA-lipid complex, DNA-CTMA. Mix DNA and CTMA solutions withstirring for several hours. The white DNA-CTMA precipitates will visiblyappear and will accumulate in the mixture;

iv. Filter the DNA-CTMA precipitates out of the mixture and rinse itcompletely;

v. Place the DNA-CTMA precipitates in a beaker and then place it in anoven at ˜40° C. to dry the DNA-CTMA precipitates overnight;

vi. Make butanol-based DNA-CTMA solution: dissolve DNA-CTMA precipitatein butanol using a tumbler or a stirrer for several hours until it fullydissolved.

Making BESM films is described in greater detail above and summarizedbelow:

i. Make BESM by mixing metal and carbon nanoparticles and nanopowders atthe desired ratio (wt %) with the DNA-CTMA solution using a stirrer orsonicator;

ii. Cast BESM on substrates, such as glass or plastic slides and drythem at room temperature to form BESM films on the substrates. BESM filmthickness is typically around 30-50 μm.

Experiments and Results

The DNA-based BESM samples were tested for their EMI shieldingeffectiveness under a very wide frequency range from KHz to tens of GHz.To cover such wide testing frequency range, the testing was divided intotwo frequency ranges, one of which was from sub-HF to UHF bands (tens ofKHz to ˜1 GHz) and the other was for X and Ku bands (6-18 GHz). Twodifferent test setups were employed for these two frequency ranges,respectively. More detail with respect to the testing is set forth inthe paper De Yu Zang, James Grote, “DNA-based nanoparticle compositematerials for EMI shielding”, in RF and Millimeter-Wave Photonics II,Robert L. Nelson; Dennis W. Prather; Chris Schuetz; Garrett J.Schneider, Editors, Proceedings of SPIE Vol. 8259 (SPIE, Bellingham, W A2012), 825908 which is incorporated herein in its entirety.

Test Setups

Test setup for High Frequency (X and Ku Bands)

The setup was based on direct and open-air measurements that consistedof an RF-isolating chamber, two horn-antennas, RF generator and spectralanalyzer. The RF-isolating chamber (˜19″×26″×40″ inner space) wasconstructed with RF absorber boards, which were thick (˜2 inches)multiple-layered, conductive plastic sponges covered with aluminumfoils. The boards could block or absorb up to 30 dB RF radiation. Thus,the RF-isolating chamber could significantly reduce the RF disturbanceoutside and inside the chamber during the measurements. A pair of WR 90horn-antennas (Advanced Technological Materials Co.) were used as the RFemitter and receiver, respectively. The RF generator and spectralanalyzer were: HP 8341B RF generator (frequency range: 10 MHz-20 GHz)and HP 71200C RF spectral analyzer.

Two antennas were separated by ˜15 cm, which was ˜five times that of theRF wavelength (˜3 cm at 10 GHz). Thus, the RF wave between the twoantennas can be considered as a far field propagation. The BESM samplewas located in the middle between the emitter and receiver.

Test Setup for Low Frequency (From Sub-HF to UHF Bands)

The test setup for low frequency testing was an indirect measurementusing an electro-optic (EO)-based, electric (E)-field sensor. The setupconsisted of an E-field sensor, an E-field generator, RF-generator andspectrum analyzer. In the experiments, the external E-field wascontrolled under MV/cm to keep the system in a linear regime, withinwhich the system linearly responded to the RF power applied.

a) The EO E-Field Sensor

The EO E-field sensor is referred to as a slab coupled optical-fibersensor (SCOS), constructed with an EO crystal and two specially-treatedoptical fibers. The fibers were pigtailed with the tiny piece of EOcrystal, Potassium Titanyl Phosphate (KTP), which is an E-field sensingarea, as shown in FIG. 2 (A). In the operation, the light beam wascoupled into one optical fiber, passed through the EO crystal and thencoupled out of another fiber. The light intensity would be altered bythe presence of an external electric field in the EO crystal area viaPockels effect.

b) E-Field Generator and Test Scheme

A small and simple E-field generator formed by two parallel metal plates(˜2.5″×2.5″) was used in the experiments. In the operation, RF signalswere applied to the E-field generator to create a strong E-field in thegap between the two parallel plates, where the EO E-field sensor wasplaced. As the RF was applied, the E-field generator create an E-fieldon the EO sensor and the light intensity in the EO sensor was altered bythe E-field.

A pulse/function generator, HP 8111A, was employed to apply AC voltageto the E-field generator. The modulated light signals from the E-fieldsensor were detected by a photodetector (PD) and converted back toelectrical signals, which were analyzed by an RF spectrum analyzer (HP8656B). The system had linear response to the RF power if the e-fieldapplied on the sensor was small enough (E<10⁶ V/m). Therefore, thee-field was controlled to be not greater than 2×10⁴ V/m for theexperiments. Using this test setup, the EMI shielding effectiveness ofBESM films could then be measured by comparing the strengths of themodulated light signals from the sensor with/without the BESM films inplace.

BESM Samples

Most of the DNA-based BESM samples were made on glass slides (1″×3″,typically) with a BESM film thickness of ˜30-50 μm, except for a fewwhich were made on Scotch tape. BESM glass samples were used formeasurements in the high frequency ranges, while the BESM-Scotch tapesamples were used in low-frequency ranges. The flexible BESM-Scotch tapesamples could be easily wrapped on the E-field sensor

As an example, but not limited thereto, twelve typical BESM samples onglass slides are listed in Table 2. They could be divided into twogroups: a) single metal powders (S1-S4, S11 and S12) and b)multiple-metal powders (S5-S10). These samples were prepared for themeasurements at high-frequencies (6-18 GHz).

Two BESM samples on Scotch tape were made with 4% and 8% Ag-dopingratios. Both samples were nonconductive and were prepared for themeasurements at low-frequencies ranging from tens KHz to ˜1 GHz.

TABLE 2 Typical BESM Samples DNA Concentration Doper Concentration inbutanol in DNA-CTMA Surface Samples Dopers (wt %) (wt %) Conductivity S1Graphite #1¹ ~10 10% No S2 Graphite #2² ~10 10% Yes (at large areas) S3Fe ~10 20% No S4 Ni ~10 20% No S5 Ag + Brass ~10 Ag: 5.3%, Brass: No7.8% S6 Ag + Cu ~10 Ag: 5%, Cu: 5% No S7 Ag + Ag—Al³ ~10 Ag: 5%, Ag—Al:5% Yes (at some small areas) S8 Ag + Fe ~10 Ag: 5.3%, Fe: 6.5% No S9Ag + Graphite ~10 Ag: 5%, Graphite: No #1¹ 5.7% S10 Ag + Mumetal⁴ ~10Ag: 5%, Mumetal: Yes (at some 5% small areas) S11 Mumetal⁴ ~10 Mumetal:10% No S12 Ni ~10 Ni: 10% No

Test Results

Results from High Frequency Measurements

For example, but not limited thereto, twelve BESM samples listed inTable 2 above were tested for EMI shielding at frequencies of 6-18 GHz.These samples could be basically divided into two groups based on thetypes of the metal dopants: Group 1 (G1) with one single metal dopant;and Group 2 (G2) with multiple-metal dopants (silver plus other metals).In G1 group, single metal dopants included C #1 (synthetic conductivegraphite), C #2 (natural microcrystal graphite), Ni, Fe, and Mumetalpowders. In the G2 group, the dopants were: Ag-C #1, Ag-Brass, Ag—Fe,Ag—(Ag—Al), Ag—Ni and Ag-Mumetal. The shielding effectiveness of the G2samples were generally better than that of most of the samples in the G1group (except S1 and S2, which were compatible with that in G2 samples).

The test results are summarized in Figure X, in which the EMI shieldingeffectiveness of BESM films was plotted as a function of RF frequencies.The data in this figure are divided into two groups. The data points inone group were obtained from the measurements of samples in G2 and S1and S2 in G1, while data in the other group were obtained from themeasurements of four samples in G1 (S3, S4, S11 and S12). The dopants ofthese eight samples in the red circle were silver-based multiple-metaland carbon-based graphite powders, C #1 and C #2, respectively. Thesesamples demonstrated impressive EMI shielding effectiveness over afrequency range from 6 to 18 GHz. In contrast, the data points in theblue circle, obtained from measurements of S3, S4, S11 and S12 in G1with dopants of Iron, Ni, and Mumetal, showed relatively poor EMIshielding effectiveness.

Results from Low Frequency Measurements

For low frequency measurements, an indirect measurement technique wasused based on EO E-field sensor, as mentioned above. Two silver-dopedBESM samples on Scotch tape (SC1: Ag 4% and SC2: Ag 8%) were made forthe measurements. Although the measurements were mostly limited by thesystem's signal-to-noise-ratio (SNR) in the frequency range below 200MHz, their shielding effectiveness was still quite impressively high andit could reach up to >60 dB at ˜100 MHz. At frequencies above 200 MHz,the shielding effectiveness was somehow affected by the RF leakage fromthe system; however, the shielding effectiveness was still mostly above30 dB. Compared with the test results of both samples, there was not anysignificant difference between them, indicating the doping concentrationof 4% may be sufficient for EMI shielding purpose.

Metal fillers are responsible for the EMI shielding effects in eitherthe conductive or nonconductive BESM and the conductivity of the fillersis utilized for high EMI shielding efficiency. Generally, the moreconductive of the filler is, the better the EMI shielding effectivenesswould be. However, some very high conductive metal powders, such ascopper, did not exhibit EMI shielding efficiently when they were used asfillers in the BESM. The reason may be due to the fact that the coppernanopowders are easily subject to oxidation when exposed to air.Oxidized copper is not a good conductor, therefore, the copper-dopedBESM is generally not good for EMI shielding.

Shielding efficiency increased with increasing filler dopingconcentration (at small doping concentrations). However, it could veryquickly reach the saturation point of doping concentration.

The biopolymer-based materials as defined in these embodiments haveexhibited high EMI shielding effectiveness while being nonconductive.The nonconductive BESM may have a great potential in themicroelectronics industries for EMI shielding applications on electronicdevices and circuit boards.

Although the invention has been described with respect to variousembodiments, it should be realized this invention is also capable of awide variety of further and other embodiments within the spirit andscope of the appended claims.

What is claimed is:
 1. A method of making an EMI shielding material, themethod comprising: dissolving DNA in a first amount of de-ionized waterfor a first period of time to form a DNA solution; dissolving CTMA in asecond amount de-ionized water for a second period of time to form aCTMA solution; making a DNA-CTMA complex by adding the DNA solution tothe CTMA solution, thereby causing a DNA-CTMA precipitate to form;making an alcohol based DNA-CTMA solution by dissolving the DNA-CTMAprecipitate in alcohol; and making the EMI shielding material by mixingnano-powders or nanoparticles, the nano-powders or nanoparticles beingone of metal or semiconductor nano-powders or nanoparticles, with thealcohol based DNA-CTMA solution.
 2. The method of claim 1, wherein atemperature of the first and second amounts of de-inonized water isabout 60° F. to about 80° F.
 3. The method of claim 1, wherein a ratioof DNA to de-ionized water is about 4 grams/liter.
 4. The method ofclaim 1, wherein the first period of time is about two hours.
 5. Themethod of claim 1, wherein a ratio of CTMA to de-ionized water is about4 grams/liter.
 6. The method of claim 1, wherein the second period oftime is about five minutes.
 7. The method of claim 1, wherein adding theDNA solution to the CTMA solution comprises adding the DNA solutiondrop-wise to the CTMA solution.
 8. The method of claim 7, wherein a rateof adding the DNA solution drop-wise to the CTMA solution is about 1drop per minute.
 9. The method of claim 1, wherein the nano-powders ornanoparticles are neutral in charge.
 10. The method of claim 1, whereinthe nano-powders or nanoparticles are metal or carbon based nano-powdersor nanoparticles-.
 11. A method for scattering an EMI signal, the methodcomprising: using the EMI shielding material made by the method ofclaim
 1. 12. A method for shielding of electromagnetic radiation andradar stealth, the method comprising: using the EMI shielding materialmade by the method of claim
 1. 13. The method of claim 1, wherein thenano-powders or nanoparticles are positively charged.
 14. The method ofclaim 1, wherein the nano-powders or nanoparticles are negativelycharged.
 15. A method of making an EMI shielding material, the methodcomprising: dissolving DNA in a first amount of de-ionized water for afirst period of time to form a DNA solution; mixing positively chargednano-powders or nanoparticles, the nano-powders or nanoparticles beingone of metal or semiconductor nano-powders or nanoparticles, to the DNAsolution, in order to make a DNA-nano-powder or nanaprticle solution;dissolving CTMA in a second amount de-ionized water for a second periodof time to form a CTMA solution; making a DNA-CTMA-metal orsemiconductor nanopowder or nanoparticle complex by adding theDNA-nano-powder or nanaprticle solution to the CTMA solution, therebycausing a DNA-CTMA-nano-powder or nanparticle precipitate to form;making an alcohol based DNA-CTMA-nanopowder or nanoparticle EMIshielding material solution by dissolving the DNA-CTMA-nanopowder ornanoparticle precipitate in alcohol.
 16. The method of claim 15, whereina temperature of the first and second amounts of de-inonized water isabout 60° F. to about 80° F.
 17. The method of claim 15, wherein a ratioof DNA to de-ionized water is about 4 grams/liter.
 18. The method ofclaim 15, wherein the first period of time is about two hours.
 19. Themethod of claim 15, wherein a ratio of CTMA to de-ionized water is about4 grams/liter.
 20. The method of claim 15, wherein the second period oftime is about five minutes.
 21. The method of claim 15, wherein addingthe DNA-nano-powder or nanaprticle solution to the CTMA solutioncomprises adding the DNA solution drop-wise to the CTMA solution. 22.The method of claim 21, wherein a rate of adding the DNA-nano-powder ornanaprticle solution drop-wise to the CTMA solution is about 1 drop perminute.
 23. The method of claim 15, wherein the nano-powders ornanoparticles are metal or carbon based nano-powders or nanoparticles.24. A method of making an EMI shielding material, the method comprising:dissolving DNA in a first amount of de-ionized water for a first periodof time to form a DNA solution; dissolving CTMA in a second amountde-ionized water for a second period of time to form a CTMA solution;mixing negatively charged nano-powders or nanoparticles, thenano-powders or nanoparticles being one of metal or semiconductornano-powders or nanoparticles, to the CTMA solution, in order to make aCTMA-nano-powder or nanaprticle solution; making a DNA-CTMA-nano-powderor nanoparticle complex by adding the DNA solution to CTMA-nano-powderor nanaprticle solution, thereby causing a DNA-CTMA-nano-powder ornanparticle precipitate to form; making an alcohol basedDNA-CTMA-nano-powder or nanoparticle EMI shielding material solution bydissolving the DNA-CTMA-nano-powder or nanoparticle precipitate inalcohol.
 25. The method of claim 24, wherein a temperature of the firstand second amounts of de-inonized water is about 60° F. to about 80° F.26. The method of claim 24, wherein a ratio of DNA to de-ionized wateris about 4 grams/liter.
 27. The method of claim 24, wherein the firstperiod of time is about two hours.
 28. The method of claim 24, wherein aratio of CTMA to de-ionized water is about 4 grams/liter.
 29. The methodof claim 24, wherein the second period of time is about five minutes.30. The method of claim 24, wherein adding the DNA solution to theCTMA-nano-powder or nanaprticle solution comprises adding the DNAsolution drop-wise to the CTMA solution.
 31. The method of claim 30,wherein a rate of adding the DNA solution drop-wise to theCTMA-nano-powder or nanaprticle solution is about 1 drop per minute. 32.The method of claim 24, wherein the nano-powders or nanoparticles aremetal or carbon based nano-powders or nanoparticles.